FIG. 7 shows a typical liquid crystal display element of a TN (twisted nematic) mode using conventional spacers.
This liquid crystal display element comprises a pair of substrates 37, 39, spacers 38 disposed between the pair of substrates 37, 39 so as to maintain a constant cell gap therebetween, a nematic liquid crystal 41, a sealing material 30 filled in the periphery of the cell gap between the pair of substrates 37, 39, and polarizing sheets 42, 43 coated on the surfaces of the respective substrates 37, 39.
The above-mentioned substrates 37, 39 are formed by patterning transparent electrodes 32, 35 made of ITO (Indium-Tin-Oxide) films on one surface of respective transparent substrates 31, 34 made of glass and by coating the surfaces of the transparent electrodes 32, 35 and the transparent substrates 31, 34 with alignment coat (polyimide films). The alignment coat 33, 36 are provided with alignment control by rubbing.
The spacers 38 are made from inorganic materials including aluminum oxide, silicon dioxide and the like, or synthetic resin materials including benzoguanamine, polystyrene type polymer and the like. The spacers made from inorganic materials are disclosed, for example, in Japanese Laid-Open Patent Publication Nos. 63-73225 and 1-59974, and the spacers made from synthetic resin materials are disclosed in Japanese Laid-Open Patent Publication Nos. 60-200228 and 1-293316.
The liquid crystal display element with the above-mentioned structure is usually produced as follows.
The spacers 38 are dispersed on the alignment coat 33 of the substrate 37, and resin for sealing is coated on the periphery of the substrate 37 by printing. Then, the pair of substrates 37, 39 are superimposed so that the alignment coat 33, 36 are faced to each other and pressed. The resin for sealing is hardened by heating to form a sealed material, thereby fixing the pair of substrates 37, 39 to each other. The nematic liquid crystal 41 is filled in the space between the pair of substrates 37, 39 through a hole provided in the sealing material, and after that, the hole is closed. Then, the polarizing sheets 42, 43 are layered on the outside surfaces of the transparent substrate 31, 34.
As spacers used for the above-mentioned liquid crystal display element, colored spherical spacers are often used for the following reasons.
In the liquid crystal display element, the liquid crystal is optically changed to form an image by applying a voltage between the transparent electrodes. However, spacers are not optically changed by the application of a voltage. Therefore, uncolored spacers are likely to be observed as white spots in dark portions of a displayed image, resulting in a decrease in the contrast of the image display.
Colored spherical spacers made from inorganic materials are disclosed in Japanese Laid-Open Patent Publication Nos. 62-66228, 63-89408, and 63-89890. Colored spherical spacers made from synthetic resin materials are disclosed in Japanese Laid-Open Patent Publication Nos. 1-200227, 1-207719 and 2-214781.
Moreover, the spacers having no adhesion are not fixed to the transparent substrates, giving rise to the following disadvantages. Therefore, the adhesive coated spherical spacers are often used.
1) Air blown onto or air suction from the substrates in the process of assembling the liquid crystal display cell may cause scattering of spacers disposed on the substrates, resulting in loss of the spacers.
2) The spacers may be displaced on the surfaces of the substrates in the process of injecting liquid crystal into the liquid crystal display cell, resulting in bias of the spacer arrangement on the substrates.
3) The spacers may be displaced by the electrical or hydrodynamic forces arising while the liquid crystal display cell is in operation.
4) The spacers may be displaced when mechanical vibration acts on the liquid crystal display cell from outside.
These displacements of the spacers in the liquid crystal display cell decrease the cell gap precision and remarkably deteriorate the image quality.
Adhesive spherical spacers are disclosed, for example, in Japanese Laid-Open Utility Model Publication No. 51-22453, Japanese Laid-Open Patent Publication Nos. 63-44631, 63-94224, 63-200126, 1-247154, 1-247155 and 2-261537.
However, when the spacers made from conventional inorganic materials or spacers made from the synthetic resin materials are used as spacers for the liquid crystal display element, the following disadvantages arise.
As shown in FIG. 8, when the liquid crystal display element is produced by using the inorganic spacers 38, the spacers 38 are so hard that they damage the alignment coat 33 when both substrates 37, 39 are pressed. On a damaged portion 33a of the alignment coat 33, a desired molecule arrangement of the liquid crystal 41 cannot be maintained. For example, in the transmission type liquid crystal display element, the damaged portion 33a appears as a display defect.
Furthermore, the inorganic spacers 38 are hard to be deformed, so that the spacers 38 come into contact with the inner surfaces of the substrates 37, 39 at one point, respectively. As a result, the spacers 38 are likely to be displaced in the space including the liquid crystal 41 due to gravity or minute vibration. This disadvantage often appears in a large liquid crystal display element used for lap-top type personal computers or word processors, wall-mounting TV sets, etc. which have rapidly come into wide use in recent years, since the display surface is used in the vertical direction or the inclined direction. For example, most of the spacers 38 move downward in the liquid crystal display element, causing nonuniformity in the thickness of the liquid crystal layer and making it difficult to provide a clear image. Also, the movement of the spacers 38 damage the alignment coat 33, causing the above-mentioned display defect.
On the other hand, when spacers which are too soft are used, the following disadvantages arise. 30. It is impossible to uniformly disperse the spacers 38 on the surfaces of the substrates 37, 39, and considerable irregularity is caused in the dispersion density. When the pair of substrates 37, 39 are pressed while facing each other, the pressure applied on one spacer 38 in the small dispersion density area is large, so that the spacer 38 is largely deformed. In contrast, the pressure applied on one spacer 38 in the large dispersion density area is small, so that the spacer 38 is hardly deformed. In this way, as shown in FIG. 9, the irregularity of the dispersion density of the spacers 38 causes nonuniformity in the thickness of the liquid crystal layer disposed between the pair of the substrates 37, 39. As a result, a clear image cannot be obtained.
Moreover, when the pair of substrates 37, 39 are pressed, it is actually impossible to apply uniform pressure onto the entire substrates 37, 39, and the substrates 37, 39 are applied with different pressures at different portions thereof. Accordingly, when the spacers 38 which are too soft are used, the respective spacers 38 are deformed to different degrees because of the difference of pressure with which the spacers 38 are subjected, resulting in the nonuniformity of thickness of the liquid crystal layer. As a result, the image quality is remarkably deteriorated.
On the other hand, in the field of electronics mounting, metal particles such as Au, Ag and Ni are mixed with a binder resin to prepare conductive paste, and this paste is filled between a pair of fine electrodes, whereby the pair of fine electrodes are connected to each other. However, such metal particles are nonuniform in shape and have a larger specific gravity compared with the binder resin, so that it is difficult to disperse those metal particles in the binder resin.
In order to overcome those disadvantages, Japanese Laid-Open Patent Publication No. 59-28185 discloses that the surfaces of particles such as glass beads, silica beads, and glass fibers, the particle sizes of which are relatively uniform, are plated with metal to form conductive fine spheres. However, the conductive fine spheres disclosed in the above have core fine spheres which are too hard, and they are difficult to be deformed by compressing. Because of this, when the electrodes are connected to each other by using the above-mentioned conductive fine spheres, the contact areas between the conductive fine spheres and the surfaces of the electrodes are not spread, making it difficult to reduce contact resistance.
Japanese Laid-Open Patent Publication Nos. 62-185749 and 1-225776 discloses conductive fine spheres using polyphenylenesulfide particles, phenol resin particles and the like as core fine spheres. However, the conductive fine spheres using such synthetic resin particles as core fine spheres do not have sufficient recoverability after being deformed by compressing. Because of this, when the electrodes are connected to each other by using these conductive fine spheres and the compression load acting on both electrodes is removed, non-contacting state is formed between the conductive fine spheres and the surfaces of the electrodes, resulting in imperfect contacts.